With contemporary technology, medical and/or chemical samples can be analyzed by a wide variety of methods. However, if an analysis of diverse chemical constituents and/or spatial distributions of such constituents are desired of a sample, the method most widely used is the practice of magnetic resonance.
In the practice of magnetic resonance phenomena, RF radiation is applied to a sample by a surrounding structure and resulting resonant signals are induced in the same or another surrounding structure for analysis. The structure may be a helical coil, saddle coil, resonant cavity, or a birdcage resonator. The latter structure is the object of the present work, wherein it is desired to obtain selected resonant frequencies in a birdcage type structure to facilitate studies of large or small samples.
In general, birdcage coils are constructed to act as either a low pass or a high pass structure. As illustrated in FIG. 1A, a conventional low pass birdcage coil provides at least one capacitive element 2 electrically coupled along each conductive leg 4. In contrast, a conventional high pass birdcage coil provides a capacitive element 2 electrically coupled between each conductive leg 4 as illustrated in FIG. 1B. With both structures, the conductive legs are typically supported by, or deposited upon a non-conductive material.
Functionally, the bird cage structure may be regarded as a periodic structure which closes on itself Periodic elements of the structure produce phase shifts which must aggregate to some integer multiple of 2.pi. when summed over the closed loop.
Geometrically, the resonator has cylindrical symmetry and it is desired that the RF current in each leg be proportional to sin k.theta./2.pi. and/or cos k.theta./2.pi., where .theta. is the azimuthal angle about the cylindrical axis and k is an integer defining resonant mode. The mode k=1 provides a uniform RF field distribution within the coil structure. Quadrature operation of the coil is realized when two RF drives of with relative phases of .pi./2 are connected to the coil at two points displaced in phase by .pi./2 about the phase distribution along the periphery of the coil.
The birdcage coil is tuned as closely as possible to the desired frequency by adjusting the capacitive elements (2 in FIG. 1) equally. In current designs, the final tuning adjustment is achieved by the discrete tuning of a single capacitor within the birdcage structure. However, by adjusting only a single capacitor, the user will only be able to obtain the desired frequency without maintaining the electrical symmetry of the structure. Therefore, optimum RF distribution will not be achieved within the structure.
Even if the birdcage coil can be adjusted with all capacitive elements 2 equal in the absence of a sample or load, insertion of a sample or load will require tuning the structure yet again to compensate for a decrease in frequency caused by the inherent dielectric properties of a typical load or sample. As illustrated in FIG. 2A, a change in frequency due to introduction of a load results in a phase error which must be compensated by the single tuning capacitor to restore the correct total phase shift of .pi./2. The result of phase error is an inhomogeneous B1 field as appears in the corresponding FIGS. 2B through 2D. It is sometimes useful to be able to tune a birdcage structure for observation of different nuclear species, e.g., .sup.1 H and .sup.19 F. A tuning adjustment between such resonant frequencies could not be accomplished with variation of a single capacitance without completely destroying the uniformity of the RF field.
It would be advantageous to develop a birdcage structure which maintains the symmetry of the azimuthal distribution of phase shift around the structure as it is tuned to accommodate different loads or to observe different nuclear species. The desired structure provides a homogeneous field under all tuning conditions and preserves the symmetry necessary for quadrature operation.